This invention relates to a communication system for transmitting and receiving communication data together with signature data attached thereto for verifying the communication data. 2. Related Art
In recent years, there has been considered introduction of a system that is operative to exchange vehicle information, such as a vehicle location, a vehicle velocity, a travelling direction, and ON/OFF operation of a brake, with peripheral vehicles through inter-vehicle communications to notify the driver of one's own vehicle of a risk of collision with one or more of the peripheral vehicles, thereby avoiding an upon-meeting collision at a blind intersection before the collision occurs. Japanese Patent Application Publication No. 2009-081524 discloses such a system.
In the above system, one of the main objects is how to assure validity of communication data. There is known as a solution an electronic signature scheme based on the public key cryptosystem.
Specifically, according to the electronic signature scheme, verification data A (e.g., a message digest generated by a hash function) generated from the communication data to be verified is encrypted with a private key in the public key cryptosystem to generate encrypted data. The communication data is transmitted to a communication partner together with signature data attached thereto including the encrypted data, a public key used to decrypt the encrypted data, an electronic certificate to verify the public key. Upon reception of the signature data, the communication partner attempts to verify the received public key with the received electronic certificate, and then restores the verification data A by decrypting the encrypted data with the public key that has been verified. It is checked whether or not the received communication data has been tampered with by comparing the restored verification data A with verification data B generated from the received communication data. The received communication data is determined to be valid if the verification data A, B coincide.
In the electronic signature scheme based on the public key cryptosystem, however, a larger public key is required to assure higher reliability, which leads to larger signature data.
There exists a following disadvantage in the above scheme. In cases where a relatively small amount of communication data is allowed to be transmitted/received in each communication and is comparable in data amount to the signature data, an overhead for the signature data is increased, which leads to reduction in communication efficiency (see FIG. 11).
In particular, in the above inter-vehicle communication system, regular (or periodic) exchange of data is required. In addition, exchanged data (i.e., the vehicle information) in each communication is assumed to be about 100 bytes in size, while the signature data is 200 bytes or more in size. That is, when the signature data is attached to the data to be exchanged, a total amount of data required for each communication will be increased two or three fold.
An amount of data that can be exchanged in each communication (i.e., size of one communication frame) is determined by a baud rate and an allowable number of concurrent communication partners and other conditions.
A radius of an inter-vehicle communication area is here assumed to be 200 m, whose center is located at a traffic intersection of 4 roads each having 3 lanes on one side. Assuming that a plurality of vehicles are running spaced apart from each other with an average inter-vehicle distance of 10 m, there exist 20 vehicles per lane within a 200 m long segment of each road. Therefore, 3 lanes on one side×2 lanes on the other side×4 directions×20 vehicles per lane gives 480 vehicles within the area of 200 m radius, of course, which may depend on a traffic condition.
Assuming that each road is a high-speed way where the vehicle speed is 30 m/sec, and taking into account potential data missing due to communication errors or the like, a data transmission cycle is preferably limited to 100 ms or less such that the data missing effect on the communication control is made allowable. In addition, it may be assumed that the baud rate is around 10 Mbps, but which depends on an allocated bandwidth of radio waves.
Under such a condition, an amount of data that can be transmitted in each communication can be estimated from the following equation.10×106 [bps]×0.1 [s]/480 [vehicles]=2083 [bits]
That is, even in the absence of transmission loss, the data amount can take nothing more than about 260 bytes. In addition, an increasing packet density leads to significant reduction in communication efficiency due to frequent occurrence of packet collisions. Therefore, it is said that the data amount is actually limited to under about 30% of 260 bytes.
That is, a disadvantage of the above inter-vehicle communication system is that since the signature data is about 200 bytes in size, it is probably impossible to transmit even 100 bytes of data that are originally intended to be transmitted.
To overcome the disadvantage, the communication system can be improved as follows. The transmitter in the system generates the signature data from the verification data generated from a unit of communication data consisting of M pieces of communication data to be transmitted, and then transmits the communication data together with divided signature data where M pieces of divided signature data are generated by dividing the signature data into M pieces, and are transmitted, attached to M corresponding pieces of communication data. The receiver in the system reconstitutes the unit of communication data from M pieces of received communication data, and reconstitutes the signature data from M pieces of received divided signature data. The communication data can be verified for each unit of communication data with the verification data extracted from the reconstituted signature data.
Conventionally, higher level applications are often installed in the system on the assumption that communication errors often occur. Therefore, even in the presence of data missing, various controls on the reception-side can be continued on the basis of a data-complementing process using previously received communication data or the like.
The data-complementing process, however, cannot be applied to the reconstitution of the verification data when partial data missing occurs. Therefore, in cases where the signature data is transmitted in a divided form as described above, the original signature data (and thus the original verification data) cannot be reconstituted unless a full set of M pieces of received divided signature data are safely received.
Accordingly, when one communication frame is missing on the reception-side, the signature data (and thus the verification data) cannot be reconstituted. As a result, the entire unit of communication data (and thus M pieces of received communication data) becomes impossible to be verified.
In consideration of the foregoing, exemplary embodiments of the present invention are directed to providing a communication system for transmitting and receiving communication data together with signature data attached thereto in a divided form for verifying the communication data, which is capable of verifying the communication data even in the case of partial data missing due to communication errors.